Axillary infrared thermometer and method of use

ABSTRACT

A radiation detector for axillary temperature measurement comprises a wand having an axially directed radiation sensor at one end and an offset handle at the opposite end. The radiation sensor is mounted within a heat sink and retained by an elastomer in compression. The radiation sensor views a target surface through an emissivity compensating cup and a plastic film. A variable reference is applied to a radiation sensor and amplifier circuit in order to maintain full analog-to-digital converter resolution over design ranges of target and sensor temperature with the sensor temperature either above or below target temperature.

RELATED APPLICATIONS

This application is a Continuation of Ser. No. 09/498,279 filed Feb. 4,2000, now U.S. Pat. No. 6,241,384, which is a Continuation of Ser. No.09/253,970 filed Feb. 22, 1999, now U.S. Pat. No. 6,045,257, which is aDivisional of Ser. No. 08/738,300 filed Oct. 25, 1996, now U.S. Pat. No.5,874,736, the entire teachings of which are incorporated herein byreference.

BACKGROUND OF THE INVENTION

Neonates are incapable of maintaining their own body temperature duringthe first few weeks of life. Skin perfusion rates are very high and theinfant loses heat rapidly. Thermal management is critical, requiring anaccurate, fast, noninvasive method of core temperature measurement.

Rectal temperature has long been considered to be the standard indicatorof neonate core temperature. However, since temperature measurementsfrom different locations on a neonate's skin are sufficiently uniform asto be relatively interchangeable with one another, the clinician mayselect the most noninvasive and convenient site at which to measuretemperature. Due to its inherent safety and long established efficacy,axilla (underarm) is the most recommended site for neonates.Unfortunately, conventional thermometers such as glass/mercury,electronic and paper strip thermometers require up to several minutes toobtain an accurate axillary reading.

In recent years, infrared thermometers have come into wide use fordetection of temperature of adults. For core temperature readings,infrared thermometers which are adapted to be inserted into thepatient's ear have been extremely successful.

Infrared ear thermometry has not found such high acceptance for use withneonates. Neonates have a very high moisture level in their ear canals,due to the presence of vernix and residual amniotic fluid, resulting inlow tympanic temperatures because of the associative evaporativecooling. In addition, environmental uncertainties, such as radiantheaters and warming pads can significantly influence the airtemperature. Further, clinicians are less inclined to position the tipof an infrared thermometer in the ear of a small neonate.

An infrared thermometer designed for axillary temperature measurementsis presented in U.S. patent application Ser. No. 08/469,484. In thatdevice, an infrared detector probe extends from a temperature displayhousing and may easily slide into the axilla to lightly touch the apexof the axilla and provide an accurate infrared temperature reading in aslittle as one-half second.

The axillary infrared thermometer has found great utility not only withneonates but as a screening tool in general and especially for smallchildren where conventional temperature measurements such as athermometer under the tongue or a rectal thermometer are difficult. Theaxillary measurement is particularly accurate with children where thereis a lack of axillary hair and perspiration.

SUMMARY OF THE INVENTION

The present invention has particular applicability to an axillaryinfrared thermometer which is suitable for nonclinical use but whichretains the accuracy required for clinical use.

In accordance with one aspect of the invention, the thermometercomprises an extended housing forming a wand which has a handle portionat one end and a radiation detection portion at the other end. The wandconfiguration allows the device to be passed through the sleeve of thepatient to the axilla to minimize exposure of the patient to thesurrounding environment. Further, the housing surface is formed of a lowthermal-conductivity material to minimize cooling of the patient withcontact against the skin. The radiation detection portion of the housingcomprises an axially directed window positioned within a cup. The windowpasses radiation to an infrared radiation sensor within the housing froma field of view substantially less than the area of the cup opening.

In accordance with another aspect of the invention, the handle portionof a radiation detector housing, preferably configured as a wand,extends along an axis generally parallel to the viewing axis but offsetfrom the viewing axis.

In accordance with another aspect of the invention, the cup is designedto conduct heat from the target area in order to match the heat lossfrom the target area when the radiation detector is not in place,thereby minimizing changes in target temperature.

In accordance with another aspect of the invention, the radiation sensoris mounted within a large thermal mass within the low conductivityhousing to provide an RC time constant for change in temperature of theradiation sensor, with change in temperature to which the housing isexposed, of at least 5 minutes and preferably 25 minutes.

In accordance with another aspect of the invention, a transparentplastic film is positioned over the cup to minimize the effects ofevaporation from the target surface. The thickness of the film is atleast 1.1 mils. and is preferably 1.5 mils.

In a preferred configuration, the radiation sensor is mounted within acan for viewing a target through an infrared transparent window on thecan and through an aperture at the base of an emissivity compensatingcup. The can is mounted within a bore in a heat sink with thermalcoupling to the heat sink through a rear flange. The flange may bepressed against a shoulder by an elastomer such as an o-ring. A rear capfit to the heat sink presses against the elastomer to press the canagainst the shoulder. Preferably, the cap is an externally threaded plugwhich fits within the bore. The cap has an opening therein through whichelectrical leads to the radiation sensor pass.

In accordance with another aspect of the invention, a novel detectioncircuit is provided. A radiation sensor provides an output as a functionof difference between target temperature and sensor temperature over adesign range of target temperatures and a design range of sensortemperatures. An amplifier amplifies the sensor output and ananalog-to-digital converter generates a multibit digital output from theamplified output over a voltage range of the amplified sensor output. Areference to the radiation sensor and amplifier circuit is variable toprovide high analog-to-digital converter resolution, over the designranges of target and sensor temperatures, with sensor temperatureseither above or below target temperature. The resolution is greater thanwould be obtained with a fixed reference over fall design ranges oftarget and sensor temperatures. The variable reference provides avariable offset which may be subtracted out in digital processingcircuitry prior to digital computation of target temperature.

In the preferred detection circuit, the reference is variable to offsetthe amplified output by an offset level approximating sensortemperature. The amplified sensor output then approximates targettemperature. In an alternative embodiment, the reference is set at oneof two levels depending on whether target temperature is above or belowsensor temperature. In one implementation of that embodiment, thereference is set to one of those two levels only after the amplifiedoutput has exceeded the analog-to-digital converter range using anintermediate reference.

In each embodiment of the detector circuit, the reference level ispreferably set while also compensating for amplifier offset. Theradiation sensor is isolated from the amplifier and the amplified outputis varied either to approximate the sensor temperature or to set theamplifier output at one of three levels corresponding to the tworeference levels. Preferably, a resistor which balances the resistanceof the radiation sensor is also isolated from the amplifier during theoffset calibration.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects, features and advantages of theinvention will be apparent from the following more particulardescription of preferred embodiments of the invention, as illustrated inthe accompanying drawings in which like reference characters refer tothe same parts throughout the different views. The drawings are notnecessarily to scale, emphasis instead being placed upon illustratingthe principles of the invention.

FIG. 1 is a cross-sectional view of one embodiment of the invention.

FIGS. 2A and 2B are orthogonal side views of another embodiment of theinvention.

FIG. 3 is a side view of a preferred embodiment of the invention.

FIG. 4 illustrates use of the embodiment of FIG. 3 with an infant.

FIG. 5 is a cross-sectional view of the radiation sensor assembly in theembodiment of FIG. 3.

FIG. 6 is an electrical block diagram of one implementation of theinvention.

FIG. 7 is an electrical block diagram of an improved implementation ofthe invention.

FIG. 8 is an electrical block diagram of the preferred implementation ofthe invention.

FIGS. 9A, 9B and 9C are detailed electrical schematics of the powercontrol circuit, ambient temperature detection circuit and sensoramplification circuit, respectively, of FIG. 8.

FIG. 10A is a flow chart of the measurement process including offsetdefinition and

FIG. 10B is a flow chart of digital processing to obtain targettemperature.

DETAILED DESCRIPTION OF THE INVENTION

A description of preferred embodiments of the invention follows.

FIG. 1 illustrates a compact form of a radiation detector designed foraxillary temperature measurement. It includes a conventional thermopileradiation sensor in which the thermopile is mounted in a can 20 having awindow 22 through which the sensor views a target. As illustrated, thefield of view of the sensor is about 60° and is through a reflective cup23 having an angle of about 90°. The housing 24 in which the sensor ismounted has a handle portion 26 which extends in a direction generallyparallel to the line of sight of the radiation sensor but which isoffset therefrom. Accordingly, the sensor end 28 of the housing may bereadily slid into the axilla with the housing held to the front of thepatient. A liquid crystal display 30 provides a temperature readingafter a button 32 is pressed. The radiation detector is powered by asingle battery 34.

FIGS. 2A and 2B illustrate another embodiment in which the detectorhousing takes the form of an extended wand. The same radiation sensorviews the target through a cup 36 at a distal end of the housing 38. Thehandle portion 40 is again offset from the viewing axis of the center 20and extends along an axis which is generally parallel (i.e., withinabout 20°) to the line of sight of the radiation sensor, but in thiscase the handle portion is at a proximal end of the extended probe. Thisdesign facilitates insertion of the radiation sensor end of the housingthrough the sleeve of a patient to minimize heat loss from the patientduring the measurement. To that end, the wand is about 10-20 centimeterslong. As before, a liquid crystal display 30 provides a temperaturereading after the button 32 is pressed. Again, the radiation detector ispowered by a single battery 34.

FIG. 3 illustrates a preferred embodiment of the radiation detector.Again, the housing takes the form of an extended wand having a radiationsensor at a distal end thereof which views the target along a viewingaxis 44. A handle portion 46 is provided at the proximal end of the wandand is again offset from the viewing axis 44. Again, the handle portionextends along an axis which is generally parallel to the viewing axis44, though angled about 10° from true parallel. The handle portion 46 isjoined to the viewing portion 48 along a curved section 50 whichincludes a switch button 52. A reading is provided on a display 54.

Use of the embodiment of FIG. 3 is illustrated in FIG. 4. As can beseen, the extended probe can readily be slid into the axilla through thechild's sleeve. The offset of the handle portion enables it to be easilyheld in front of the patient.

Not only is the use of the wand through the sleeve better for thepatient in minimizing heat loss, it improves the temperature measurementby avoiding cooling of the target area. As disclosed in U.S. Pat. No.5,445,158, the actual temperature of the target surface is offset fromthe desired core temperature by an amount determined by the surroundingambient temperature. Ambient temperature controls the amount of heatloss from the target area and the thermal difference between coretemperature and surface temperature. That temperature difference can becompensated electronically by computing a core temperature T_(C) fromthe sensed target temperature T_(T) and a sensed ambient temperatureT_(A) with a known coefficient K as follows:

T _(C) =T _(A) +K(T _(T) −T _(A))

However, the degree of compensation can be minimized or even eliminatedif exposure of the target area to ambient temperature is minimized.

With the wand positioned through the sleeve, the contact of the wand tothe axilla is not visible to the clinician. Accordingly, to assure thatthe actual axilla temperature is obtained, the wand may be pivoted toscan the region, with the electronics providing a peak temperaturedetection as in U.S. Pat. No. 5,445,158. In that prior system, the peakdetection algorithm was processed as long as the button on the radiationdetector was pressed. In the present system, the button is pressed onceand the peak detection continues until the temperature stabilizes. Theprobe can be scanned with the clinician viewing the temperature displayto locate the location of highest temperature. However, since thedetector is surrounded by the target area, scanning becomes lesscritical.

FIG. 5 illustrates the sensor assembly of the radiation detector of FIG.3. A thermopile 60 is mounted within a can 62 in conventional fashion.For high stability the thermopile may be a vapor deposited thermopilesurrounded with xenon gas, but for reduced cost it is preferably asemiconductor thermopile surrounded with air. An infrared radiationtransparent window 63 is provided over a viewing opening in the can. Thecan 62 is set within a bore within a heat sink 64. A shoulder defines anaperture 66 at the base of a conical cup 68 through which the thermopileviews the target. The cup is preferably of low emissivity in order toprovide emissivity compensation as disclosed in U.S. Pat. No. 4,636,091.Preferably, the heat sink 64 in which the cup is formed is of aluminum.Alternatively, the heat sink may be of brass, nickel plated in the cupregion. With the rim pressed against the shoulder and having a closetolerance with the larger diameter of the bore an air gap 73 ismaintained about the side and front of the can.

An elastomeric o-ring 70 is positioned behind the can 62. A plug 72 isthreaded into the bore in the heat sink 64 to press the ring 70 againstthe rear flange 71 of the can and thus press the flange against ashoulder in the heat sink bore. This arrangement provides for goodthermal contact between the can and the heat sink 64 at the rear andalso makes the thermopile highly resistant to mechanical shock since theshock is only transferred through the thin rim past the shock absorbingelastomer. If the flange were rigidly clamped between metal parts, therewould be a danger of shock breaking the gas seal of the can. An opening74 is provided through the center of the plug 72 for access ofelectrical leads 75 to the thermopile can.

The heat sink assembly is mounted to the housing by sliding it into anopening at the distal end of a front housing section and retaining itwith a snap ring 79. Then the rear housing section is connected to thefront section.

The plastic housing 48 in which the sensor assembly is mounted is of lowthermal conductivity, preferably less than one hundredth that ofaluminum. The housing thermally isolates the heat sink 64 from thesurrounding environment to minimize heat flow to the heat sink. Further,the heat sink 64 is of significant thermal mass. Accordingly, the RCtime constant for change in temperature of the radiation sensor, withchange in temperature to which the housing is exposed, can be made largefor a more stable temperature reading. The thermal resistance is madehigh by the low conductivity housing, and the thermal capacitance ismade high by the large mass of the heat sink 64. That RC time constantshould be at least 5 minutes and is preferably about 25 minutes.

Past designs of infrared thermometers, such as presented in U.S. Pat.No. 4,993,419, have relied on a massive thermopile can which also servedas the heat sink. That design assured a high RC time constant forthermal conduction through the external thermal barrier to the heat sinkrelative to a thermal RC time constant for temperature response of thecold junction to heat transferred to the heat sink. The latter low RCtime constant was obtained by assuring a low thermal resistance to thecold junction using expensive high conductivity material in a speciallydesigned can/heat sink. In the present device, a design goal is to use aconventional low cost thermopile mounted in a light weight can whichdoes not provide the low thermal resistance of the prior design.Accordingly, it is important that the can be mounted to assure that allheat conduction to the thermopile be through the rear of the can whichserves as the thermal ground to the thermopile. That objective isobtained by making thermal contact to the can through the rear flangeand assuring an air space about the sides and front of the can.

Forming the emissivity compensating cup 68 in the heat sink reduces thecost of the assembly and also improves the thermal characteristics.Although the emissivity of the cup is ideally zero, it is in fact about0.1. With the cup formed as part of the heat sink, it is at thetemperature to which the can is grounded. Accordingly, any thermalemissions from the surface 68 will be at substantially the sametemperature as the cold junction and thus not be seen. The electronicscan also be calibrated to compensate for the loss of reflectance due tonon-ideal emissivity, but that calibration is affected by thetemperature of the reflective surface. By assuring that the surface 68is at the temperature to which the thermopile can is grounded, thetemperature of the surface is generally known and compensation can bemade temperature dependent.

When adapted to household use, concerns for patient cross-contaminationassociated with clinical temperature detectors is not so significant.Accordingly, the disposable radiation transparent covers used in priorinfrared thermometers, such as in Ser. No. 08/469,484, are not required.However, for purposes of accuracy of measurement, it is still importantthat a thin transparent film be provided over the viewing area. Withoutthe film 76, any evaporation from the moist axillary region would resultin a temperature reduction at the target surface and reduced accuracy inthe temperature reading. However, the film can be pressed against thetarget surface, thus trapping the moisture and preventing evaporation.The thin film quickly equilibrates to the temperature of the targetsurface for an accurate reading.

In prior infrared thermometers in which the film was not contacted tothe target, it was important that the thickness of the film beminimized. Such films have generally been described as being of lessthan 0.001 inch thickness, but thicknesses of less than 0.0005 inch havebeen preferred to minimize absorption of the radiation in the film andthe resultant heating of the film which would lead to inaccuratereadings. With the present application, where the film contacts thetarget surface, the film not only allows a change in temperature, butthe temperature is forced to change to the target temperature.Accordingly, absorption of radiation is no longer a concern. Now thelimitation in thickness is the need to obtain a quick equilibration withthe target surface. Further, since the film must be more durable inorder to obtain repeated measurements while contacting the targetsurface, a thickness of greater than 0.0011 inch is important. In thepreferred embodiment, a proper balance between durability and quickthermal response is obtained with a film thickness of about 0.0015 inch.As before, polyethylene is the preferred film material.

Accordingly, a thin plastic film 76, for example of 0.0015 inchthickness, is stretched across the cup of the heat sink 64. The filmneed only be replaced periodically with wear and may be attached to theend of the housing with a formed sleeve 78 or the like. For example, thesleeve may be injection molded with the film ultrasonically welded toit. The sleeve is sufficiently thick to serve as a thermal insulatorbetween the patient and the heat sink 64.

Electrical block diagrams for three implementations of the radiationdetector are presented in FIGS. 6-8. In each implementation, amicroprocessor 80 is at the heart of the circuit. A power controlcircuit 82 responds to activation of the button switch 84 by the user toapply power to the microprocessor and other elements of the circuit.That power is maintained until the microprocessor completes themeasurement cycle and signals the power control 82 to power down. Themicroprocessor is clocked by an oscillator circuit 86 and maycommunicate with an external source for programming and calibrationthrough communication conductors 88. The temperature determined by themicroprocessor is displayed on the liquid crystal display 100, andcompletion of the temperature processing is indicated by a beeper 102.During the measurement process, the microprocessor takes readingsthrough an internal multiplexer/analog-to-digital converter 90. Thepreferred microprocessor 80 is a PIC16C74 which includes an internal8-bit A-D converter. To minimize expense, the circuit is designed torely solely on that A-D converter.

With reference to FIG. 6, thermopile 92 provides a voltage output signalequal to the fourth power difference between target temperature and thetemperature of the thermopile cold junction, offset by voltage reference94 to the thermopile. The voltage output from the thermopile isamplified by an amplifier 96, having a gain in the order of 1000, whichalso provides an offset. Through operation of the multiplexer, themicroprocessor provides an analog-to-digital conversion of the amplifiedsensor output, of the thermopile and amplifier reference voltage 94 andof an ambient temperature provided by temperature sensor 98.

The microprocessor utilizes the measured voltage reference signal 94 todetermine and allow for electrical offsets in the circuit. Thetemperature sensor 98 is positioned to sense the substantially uniformtemperature of the thermopile cold junction, can and heat sink. Withlong thermal time constants, that temperature is also considered to bethe same as ambient temperature during the short temperature reading.The microprocessor relies on the sensed temperature T_(A) to computetarget temperature T_(T) from the relationship (T_(T) ⁴-T_(A) ⁴) towhich the thermopile output is proportional. The microprocessor may alsocompute core temperature from the core temperature relationship notedabove.

ASTM standards dictate that the infrared thermometer operate correctlyover a range of ambient temperature of 16°-40° C. and provide accuratetemperature readings of target temperatures ranging from 22°-40° C. Adesign goal of the present radiation detector was to provide accuracy of0.1° C. over the full 24 degree range of 16°-40° C. Thus, the 8-bitanalog-to-digital converter must be able to convert readings within a24° range without saturation at either end of that range. With a fullrange of 24° and 0.1° accuracy, 240 analog levels are required. The 256levels processed in an 8-bit conversion would appear sufficient.However, the input to this A-D converter actually approximates thedifference between target temperature and sensor (ambient) temperature,and that difference may be up to 24° positive or negative. Allowing forthe target temperature to be either above or below the ambienttemperature by 24° necessitates a reduction in A-D converter resolutionby one half in order to convert the full ±24° C. range withoutsaturation of the analog-to-digital converter.

One approach to obtain the 0.1° C. accuracy over the full 24°temperature range with an 8-bit converter, regardless of whether thesensor temperature is above or below target temperature, is to controlthe value of the voltage reference 94 according to the particularenvironmental conditions. For example, if the readings indicate that thetarget temperature is greater than the sensor temperature, the referenceto the thermopile can be set at zero and the converter can convert thefull 24° range of target temperature above sensor temperature withoutsaturation. If, however, the target temperature were to drop belowsensor temperature with that zero offset, the resultant negative valuewould saturate the analog-to-digital converter at the lower end. In thatcase, it is necessary to provide a sufficient voltage reference 94 tothe thermopile to assure that the decrease in thermopile output withmore negative target temperature does not bring the input to the A-Dconverter to zero. The maximum input to the A-D converter then resultsfrom a zero temperature difference and the zero input to the A-Dconverter results from the maximum temperature difference. The voltagereference is controlled by the microprocessor 80, and the microprocessoris programmed to process the different algorithms depending on therelative temperatures which dictate the voltage reference.

An alternative approach is to initially set the voltage reference 94midway between the two extreme offset references discussed above. Inthat case, so long as the target and ambient temperatures are within 12°C. of each other, the output from the amplifier 96 is within the fullrange of the analog-to-digital converter. If the microprocessor sensessaturation of the A-D converter, however, it changes the voltagereference to the high or low value accordingly. In each case, themicroprocessor processes the received data according to the referencevoltage that it applies.

FIG. 7 illustrates an alternative approach by which the output of theamplifier 96 can be maintained within the acceptable range of theanalog-to-digital converter for full 0.1° C. resolution regardless ofwhere the target and sensor temperatures fall within the designtemperature ranges and without the need for determining whether thetarget temperature is above or below the sensor temperature. In thisapproach, the sensor ambient temperature T_(A) provides the voltagereference to the thermopile. Thus, if the ambient temperature were at16° C., the minimal offset would be provided to the thermopile 92 andthe output of the thermopile would be proportional to (T_(T) ⁴-T_(A) ⁴)through the full range of possible target temperatures. On the otherhand, if the ambient temperature were at 40° C., the maximum offsetwould be provided to the thermopile and the output of the thermopilewould be proportional to that maximum offset minus a voltageproportional to (T_(T) ⁴-T_(A) ⁴). Accordingly, the microprocessorsubtracts that offset before computing target temperature. If theambient temperature is at any level between the minimum and maximumdesign temperatures, the offset applied to the thermopile is set at acorresponding value between the minimum and maximum offsets. By slidingthe offset with sensor temperature, it is assured that the output ofamplifier 96 is within the 8-bit range of the analog-to-digitalconverter, regardless of the level of the target temperature, so long asthe target temperature is within the design range. The microprocessoronly has to subtract that offset from the sensed voltage prior tocomputing target temperature from the value (T_(T) ⁴-T_(A) ⁴).

The above discussion presents a design technique which allows for thehighest resolution using the 8-bit analog-to-digital converterregardless of target and ambient temperatures within the 16°-40° C.design temperature ranges. However, the discussion has not taken intoconsideration electrical offsets in the circuitry. It is desirable tominimize the costs of the analog circuitry, but less expensive circuitelements result in substantially higher amplifier offsets. Further, theoffset itself is temperature dependent, and the high gain is applied toany changes in offset. Accordingly, with an inexpensive amplifier,changes in electrical offsets with temperature can alone exceed theavailable input range of the analog-to-digital converter.

In accordance with the approach of FIG. 8, the voltage offset to theamplifier is controlled to account for whether the target temperature isabove or below ambient temperature, as discussed above, and also tocompensate for wide variations in electrical offset to permit use ofrelatively inexpensive analog circuitry. The thermopile voltagereference is fixed at a level that assures that the thermopile output isalways an acceptable positive input to the amplifier. An auto zeroswitch 104 is included to allow for isolation of the amplifier 96 fromthe thermopile 92 during a calibration sequence. During that sequence,the microprocessor provides a pulse width modulated output on line 106which is filtered by a filter 108 to produce an offset level applied tothe amplifier circuit. With the thermopile isolated from the amplifier,the amplifier output can be adjusted using the electrical offset to setthe output of the amplifier 96 at the level dictated by theconsiderations discussed above. Specifically, in the first approach, theoutput of amplifier 96 is adjusted to one extreme of theanalog-to-digital converter range depending on whether the targettemperature is higher or lower than the sensor temperature. In themodification to that approach, the output of the amplifier 96 isinitially set at an intermediate level. In the final embodiment, theoffset applied to the amplifier 96 is selected to set the amplifieroutput at a level which approximates the ambient temperature sensed bysensor 98. In each case, the electrical offset of the amplifier isbalanced out, leaving only the offset required to keep the A-D converterwithin range.

After calibration, the thermopile is again connected into the circuitusing switch 104, and the output from amplifier 96 becomes the voltagedetermined by (T_(T) ⁴-T_(A) ⁴) plus the voltage offset which had beenset by the microprocessor to maintain full resolution within the designtemperature range. The target temperature is then computed by firstsubtracting the applied offset and then determining target temperaturewith knowledge of the sensor temperature.

FIGS. 9A-9C present a specific circuit implementation of the FIG. 8approach. The power control circuit is illustrated in FIG. 9A. When theswitch S1 is pressed, power from the battery junction J2 is appliedacross diode D1, resistor R10 and zener diode D2 to apply power toterminal VSW. Terminal VSW applies power to the remainder of the system.The battery voltage is also seen by the microprocessor through resistorR1 at control line VSWITCH. The microprocessor then applies a controlsignal to line VON which turns transistors Q2 and Q1 on to maintain VSWhigh even after switch S1 is released. The voltage across zener diode D2is applied to a voltage regulation circuit including amplifier U1A togenerate a 3.5 volt reference required by the microprocessor. A 1.75volt reference used in the amplifier circuit is also generated through atransistor Q5.

FIG. 9B illustrates the ambient temperature sensor 98 which actuallymeasures the thermopile cold junction temperature. The 3.5 voltreference from the power control circuit is applied across a thermistor110 in series with resistor R4. The temperature dependent voltage fromthe voltage divider of thermistor 110 and resistor R4 is applied to thenon-inverting input of amplifier U1C to provide a temperature indicationVTA. VTA is converted to a digital signal through the multiplexer/ADconverter 90 of the microprocessor 80.

FIG. 9C illustrates the thermopile and amplifier circuit including theoffset control. The output of thermopile 92 is applied to thenon-inverting input of amplifier 96 to produce the signal V_(SIG)applied to the multiplexer/AD converter 90. During calibration, thesignal AZ from the microprocessor through transistors Q7 and Q6 closesthe switch Q3, thus short circuiting the thermopile and isolating itfrom the amplifier 96. Included within the feedback circuit of theamplifier 96 are resistors R17 and R18 which balance the internalresistance of the thermopile and resistor R15, respectively, when thethermopile output is being amplified. During the calibration cycle thoseresistors are also shorted by switch Q4 to maintain balance.

The pulse wave modulated signal on line 106 of FIG. 8 is applied ascontrol signal PWM in FIG. 9C. That signal is low pass filtered in an RCcircuit (R27, R28, C5, C6) and applied through amplifier U1D to createan offset voltage at the output of amplifier U1D. With the thermopileisolated, the microprocessor controls that offset voltage to adjust theoutput of amplifier 96 to a desired level. Thereafter, themicroprocessor maintains that offset voltage level. That offsetcompensates for the undesirable electrical offset of the amplifier andalso creates the electrical offset needed to maintain the signal V_(SIG)within the range of the A-D converter. Thereafter, the microprocessorchanges signal AZ to open the switches Q3 and Q4 and allow detection ofthe voltage signal generated by the thermopile 92.

Operation of the microprocessor in controlling the calibration processis illustrated by the flow chart of FIG. 10A. The processing of thedetected signal on line V_(SIG) is as illustrated by the flow chart ofFIG. 10B.

As illustrated in FIG. 10A, the ambient temperature is measured at 100by converting the output V_(TA) from the sensor circuit 98 of FIG. 9B.The switches Q3 and Q4 are closed at 102. With no offset signal appliedthrough PWM, the output of the amplifier 96 is the unwanted electricaloffset which must be balanced out. That signal is converted as V_(SIG)Offset at 104. If V_(SIG) Offset is within the range of the A-Dconverter, it can be measured, and the process continues from 106. Onthe other hand, if the analog-to-digital converter saturates, it isnecessary to apply a signal PWM to add an offset to the amplifier whichbrings the V_(SIG) Offset signal to within range. Accordingly, theoffset required to change the V_(SIG) output by one-half the A-Dconverter range is applied at 108. The system loops through steps 106and 108 as required to bring the V_(SIG) Offset into a measurable range.

At 110, the PWM signal required to make V_(SIG) Offset equal to V_(TA)is calculated and applied to the circuit of FIG. 9C. At this point,V_(SIG) approximates V_(TA) so the signal should remain within rangeeven when the thermopile is switched back into the circuit.

V_(SIG) Offset is measured again at 112. The calculated V_(SIG) Offsetwas only an approximation to assure that the A-D converter remainswithin range. The critical value for later computation of targettemperature is the actual offset measured at 112.

The FET switches Q3 and Q4 are then opened at 114 and V_(SIG) ismeasured at 116. Temperature is calculated and displayed at 118 as thedetector stabilizes. Until the V_(SIG) signal stabilizes to a constantlevel, the system loops from 120 to remeasure V_(SIG).

Once the temperature reading has stabilized, the FET switches Q3 and Q4are again closed at 122 and V_(SIG) Offset is measured at 124. Thisassures an accurate target temperature calculation in the event of anydrift in V_(SIG) Offset during the measurement process. The final targettemperature is calculated and displayed at 126.

The target temperature calculations performed at 118 and 126 arepresented in FIG. 10B. The voltages V_(TA) and V_(SIG) are measured at128. A thermistor offset value stored during manufacturing calibrationis subtracted from V_(TA) at 130 and the resultant value is multipliedby 3.5 volts per 256 bits at 132 since the ambient temperature look-uptable is based on a 3.5 volt range. The look-up table is addressed at134 to obtain the ambient temperature value T_(A).

A temperature coefficient Tempco which allows for the temperaturedependence of the thermopile output is calculated at 136:

Tempco=(1+K _(T)(T _(A) −T _(Z))

where K_(T) is a stored sensor specific gain constant and T_(Z) was thetemperature of the device stored during manufacturing calibration.

At 138, the measured V_(SIG) Offset is subtracted from V_(SIG) to obtaina value which indicates the value (T_(T) ⁴-T_(A) ⁴). More specifically,Hc/s is computed at 140:

Hc/s=(V _(SIG) −V _(SIG) Offset)×Kh×(Tempco)

where

Kh=Sensor Gain Constant (Determined at calibration)

T _(T)=(Hc/s+T _(A) ⁴)^(¼)

To obtain target temperature, T_(A) ⁴ is added to Hc/s at 142 and thefourth root is obtained by table look-up at 144.

Each of the look-up tables may be modified to account for compensationssuch as core temperature compensation and temperature dependantemissivity calibration as discussed above.

While this invention has been particularly shown and described withreferences to preferred embodiments thereof it will be understood bythose skilled in the art that various changes in form and details may bemade therein without departing from the scope of the inventionencompassed by the appended claims.

What is claimed is:
 1. A radiation detector comprising: a radiationsensor which provides an output as a function of difference betweentarget temperature and sensor temperature over a design range of targettemperatures and a design range of sensor temperatures; an amplifier incircuit with the radiation sensor which amplifies the sensor output toproduce an amplified sensor output; an analog-to-digital converter whichgenerates a multibit digital output from the amplified sensor outputover a voltage range of the amplified sensor output; and a variablereference to maintain analog-to-digital converter resolution over thedesign ranges of target and sensor temperatures, the resolution beinggreater than would be obtained with a fixed reference over a full designrange of target temperature and a design range of sensor temperatures.2. A radiation detector as claimed in claim 1 wherein the reference isvariable to offset the amplified sensor output by an offset levelapproximating sensor temperature.
 3. A radiation detector as claimed inclaim 2 wherein the amplified sensor output approximates targettemperature.
 4. A radiation detector as claimed in claim 2 furthercomprising a switch to isolate the radiation sensor from the amplifier,the reference being varied to provide an amplified output whichapproximates sensor temperature while the radiation sensor is isolatedfrom the amplifier.
 5. A radiation detector as claimed in claim 4further comprising a switch to isolate a resistor which balancesresistance of the radiation sensor.
 6. A radiation detector as claimedin claim 1 wherein the reference is set at one of two levels dependingon whether target temperature is above or below sensor temperature.
 7. Aradiation detector as claimed in claim 6 further comprising a switch toisolate the radiation sensor from the amplifier, the reference beingvaried to provide an amplified output at one of the two levels while theradiation sensor is isolated from the amplifier.
 8. A radiation detectoras claimed in claim 7 further comprising a switch to isolate a resistorwhich balances resistance of the radiation sensor.
 9. A radiationdetector as claimed in claim 1 wherein a measurement offset resultingfrom the reference is subtracted from the multibit digital output priorto digital computation of target temperature therefrom.
 10. A radiationdetector as claimed in claim 1, wherein the variable reference isapplied to the amplifier.
 11. A method of detecting temperaturecomprising: amplifying the output of a radiation sensor with anamplifier and applying the amplified output to an analog-to-digitalconverter; and varying a reference to maintain analog-to-digitalconverter resolution over design ranges of target and sensortemperatures, the resolution being greater than would be obtained with afixed reference over a full design range of target temperatures anddesign range of sensor temperatures.
 12. A method as claimed in claim 11wherein the reference is varied to offset the amplified output by anoffset level approximating sensor temperature.
 13. A method as claimedin claim 12 wherein the amplified output approximates targettemperature.
 14. A method as claimed in claim 12 further comprisingvarying the reference by isolating the radiation sensor from theamplifier and, while the radiation sensor is isolated from the amplifierproducing the reference to provide an amplified output whichapproximates sensor temperature.
 15. A method as claimed in claim 14further comprising isolating a resistor which balances resistance of theisolated radiation sensor.
 16. A method as claimed in claim 11 whereinthe reference is set at one of two level depending on whether targettemperature is above or below sensor temperature.
 17. A method asclaimed in claim 16 wherein the reference is varied with the radiationsensor isolated from the amplifier to provide an amplified output at oneof the two levels.
 18. A method as claimed in claim 16 wherein thereference is initially set at an intermediate level and, if theamplified output is out of range of the analog-to-digital converter, thereference is then set at one of said two levels.
 19. A method as claimedin claim 11 wherein a measurement offset resulting from the reference issubtracted from the multibit digital output prior to digital computationof target temperature therefrom.
 20. A method as claimed in claim 11further comprising the step of varying the reference to the amplifier.